“… when you walk around a kitchen, you will say to yourself, this is interesting, this is grand, this is beautiful like Chardin.”

-Marcel Proust

Background

Titration of positive end-expiratory pressure [PEEP] in the acute respiratory distress syndrome [ARDS] is achieved by a diverse assortment of practices undergirded by equally lavish bases of rationale. Of note, the recent ART Trial has tempered enthusiasm for recruitment maneuvers and, potentially, higher PEEP. Further, prone position in ARDS has been freshly advocated – especially for patients with a PaO2/FiO2 ratio of less than 100. Yet, there is strangely little data directing how PEEP and prone position may mechanically play off of each other from the perspective of cardiothoracic mechanics. Indeed, prone position itself may alter hemodynamics in ARDS based on many interacting and conflicting physiological forces such as intra-abdominal pressure, venous return, cardiac function and thoracic mechanics.

To better study these intertwined affairs, a recent investigation employed a porcine model of ARDS in which a spectrum of PEEP values were studied across the supine and prone positions as well as with and without intra-abdominal hypertension [IAH].

What They Did

A saline lavage lung injury model was employed with 12 pigs studied in the supine and prone positions as well as with a normal abdominal pressure or with IAH [15 mmHg]. Esophageal manometry was employed to measure the trans-pulmonary pressure [Ptp] which is a surrogate for the distending pressure across the lung [i.e. airway pressure less the esophageal pressure]. This measurement also allowed for the quantification of lung and chest wall compliance. The animals were ventilated with volume controlled ventilation and a tidal volume of 15mL/kg.

Accordingly, the independent variables were two by two factorial positional [supine/prone] and abdominal pressure [normal/IAH] matrix with each of these 4 conditions exposed to 6 levels of PEEP [20, 17, 14, 11, 8, and 5 cmH2O]. With each decrement in PEEP level, a recruitment maneuver was applied [10 cm H2O inspiratory pressure above a PEEP of 20 cm H2O] for 10 breaths.

The PEEP which provided the optimal respiratory mechanics and cardiac output was recorded and compared; thus, respiratory mechanics and cardiac output were the dependent variables.

What They Found

Both PEEP and the prone position independently improved respiratory mechanics; there was no interaction or synergy between the two interventions. In other words, the same level of PEEP resulted in the largest improvement in pulmonary compliance when comparing supine to prone. Notably, intra-abdominal hypertension increased the level of PEEP required to improve pulmonary mechanics. A PEEP of 11 cm H2O resulted in the best pulmonary compliance for both supine and prone with normal abdominal pressure. By contrast, a PEEP of 17 cm H2O resulted in the best pulmonary compliance in both supine and prone positions with an elevated intra-abdominal pressure. At both of these levels of PEEP, the prone position generated greater pulmonary compliance in absolute terms. Essentially identical results were obtained for the results of PEEP on both the compliance of the respiratory system and driving pressure.

As expected, the chest wall was stiffened upon pronation and with intra-abdominal hypertension. When optimized, the ‘best’ values of chest wall compliance were, qualitatively, as follows: supine/normal > supine/IAH ≈ prone/normal > prone/IAH. Interestingly, the PEEP level at which the end-expiratory trans-pulmonary pressure became positive was similar between the supine and prone positions.

With respect to cardiac output, higher PEEP levels could be applied in the prone position to achieve the same cardiac output as compared to the supine position.

Thoughts

Firstly, these experimental data demonstrate that both PEEP and prone position independently improve pulmonary mechanics, but without multiplicative effects between them. Of note, if we consider Suter’s classic description of ‘optimal PEEP’ we should be curious about the level of PEEP that generates the best oxygen delivery [DO2]. DO2 was chosen in Suter’s study because of the trade-off between increasing arterial oxygen tension with PEEP and the decrement in cardiac output that occurs with pressurizing the thorax. Yet the mechanisms underpinning the PEEP-mediated improvement in oxygenation are manifold. PEEP recruits collapsed lung and thus improves VQ matching, yet by reducing cardiac output to non-ventilated areas, PEEP may additionally improve shunt by hemodynamic means; in other words, PEEP increases V but also lowers Q to areas of low V.

Further, the effects of PEEP are difficult to anticipate because thoracic and abdominal pressurization will affect upstream pressure for venous return [i.e. the MSFP], the downstream pressure [i.e. the CVP] as well as right ventricular and left ventricular afterload. Despite these complexities, Suter found that optimal DO2 correlated fairly well with best respiratory mechanics [respiratory system compliance].

In the porcine study at hand, the best lung mechanics were reached near a PEEP of 11 for both positions with normal intra-abdominal pressure [IAP] and roughly a PEEP of 17 for both positions with elevated IAP. Interestingly, optimal cardiac output was obtained at lower levels of PEEP for each combination of position and abdominal pressure. However, it is important to note that DO2 was not reported so it is possible that cardiac output fell when lung mechanics were optimized, but the fall in cardiac output was offset by the increase in oxygen saturation and therefore delivery; this data would have been nice.

Unexpectedly, the authors report a similar end-expiratory trans-pulmonary pressure [i.e. the pressure across the lung during end-expiration] across a range of PEEP in the supine and prone position; given that prone position stiffens the chest wall, I had expected that Ptp would be lower [i.e. higher pleural pressure] upon pronation. This discrepancy, may be explained, however, by mediastinal weight. Indeed, I am surprised that the authors did not account for mediastinal weight in the supine position given that these authors had previously published porcine data on lung injury comparing different positions [supine, semi-Fowler, prone, decubitus]. They found that from supine to prone position the esophageal pressure fell by about 2 cm H2O. If this correction had been made, the trans-pulmonary pressures would have been greaterin the supine positions because the esophageal pressures would have been lower with the effect of mediastinal weight removed. That higher PEEP predicted greater cardiac output in the prone position may, therefore, have been due to increased venous return due to abdominal pressurization and/or lower RV afterload secondary to diminished Ptp.

Ultimately, and in light of the recent ART Trial as well as evolving literature on phenotypic subsets of ARDS, it is still most advisable to approach PEEP on a case-by-case basis. Be wary when there is clinical evidence for elevated IAP and carefully utilize driving pressure, stress index and/or esophageal manometry for titrating lung mechanics in both the supine and prone positions.

Lastly, it is understandable that the discussion of PEEP may not immediately grab the reader’s interest. Perhaps this is because the application of PEEP is so commonplace that its prescription falls to the background of thought. Yet, like many well-traveled avenues of life, habit can deaden our senses and pull us away from what was once was mysterious. Let that not happen with PEEP! Walk around the ICU like it is your mundane, morning kitchen scape; but instead, look upon PEEP – and other humdrum things – as if it is a grand and beautiful impression by Chardin.